feat(deploy): add optional Spotify lyrics sidecar

docs/operations/spotify-lyrics-sidecar.md

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+# Spotify lyrics sidecar
+
+Spotify lyrics are optional. The normal and AIO installs stay healthy without this service. Add it only when you
+have a Spotify web session cookie and want Spotify to participate in the lyrics route.
+
+The sidecar is pinned by digest, stays on Allstarr's private Docker network, and does not publish a host port. Its
+upstream image currently runs on `linux/amd64`; ARM hosts need Docker's emulation support.
+
+## Add it
+
+Keep the cookie in the stack's host `.env`:
+
+```dotenv
+SPOTIFY_API_SESSION_COOKIE=replace-with-your-sp-dc-cookie
+SPOTIFY_LYRICS_API_URL=http://spotify-lyrics:8080
+```
+
+Then validate and apply the optional overlay:
+
+```bash
+docker compose -f docker-compose.yml -f docker-compose.spotify-lyrics.yml config --quiet
+docker compose -f docker-compose.yml -f docker-compose.spotify-lyrics.yml up -d
+docker compose -f docker-compose.yml -f docker-compose.spotify-lyrics.yml ps
+```
+
+This does not replace Postgres, Valkey, application state, downloads, or kept media. Compose may recreate the
+Allstarr container to attach the endpoint setting, but it reuses the same volumes.
+
+If a legacy `.env` import contains `SPOTIFY_LYRICS_API_URL`, Allstarr imports that URL as a durable runtime setting.
+The WebUI never starts containers or copies the Spotify cookie into a sidecar. The administrator must still add the
+overlay so Docker creates the service and supplies its cookie.
+
+## Update or remove it
+
+Normal updates use the same overlay:
+
+```bash
+docker compose -f docker-compose.yml -f docker-compose.spotify-lyrics.yml pull
+docker compose -f docker-compose.yml -f docker-compose.spotify-lyrics.yml up -d
+```
+
+To remove only the optional service:
+
+```bash
+docker compose -f docker-compose.yml -f docker-compose.spotify-lyrics.yml rm -s -f spotify-lyrics
+docker compose -f docker-compose.yml up -d
+```
+
+Remove or clear the Spotify lyrics URL in Sources if you do not want Allstarr to probe the absent endpoint. Removing
+the sidecar never removes music or durable Allstarr data.
+